Independently adjustable dual polarized diplexer

ABSTRACT

Microwave diplexer apparatus for handling two independent polarized transmitted signals at one frequency and two independent received signals at a lower frequency. The two transmitted signals are introduced separately into a first twoport orthomode transducer so as to be mutually orthogonal. From the orthomode transducer they pass through a polarization rotator, which can be rotated to adjust their angle of polarization, to a first four-port orthomode transducer. The transmitted signals pass through an antenna port of the four-port orthomode transducer to the antenna. Received orthogonally polarized signals from the antenna pass into the four-port orthomode transducer. They are blocked from entering the first two-port orthomode transducer since they are below the cutoff frequency of that transmission path. They pass through four ports which are positioned orthogonally in the four-port orthomode transducer. Filters at these ports prevent the high frequency transmitted signal from passing through. The received signals are conducted by four sections of rectangular waveguide to a second four-port orthomode transducer. The orthogonally polarized received signals pass through a polarization rotator to a second two-port orthomode transducer. When the rotator is properly oriented, the two polarized received signals are separately and independently coupled to two ports of the two-port orthomode transducer which are mutually orthogonal. By separately adjusting the angles of polarization by means of the two polarization rotators, the polarizations of the transmitted and received signals can be changed independently of each other without changing the physical orientation of the apparatus.

waited. gtates Paeiit 1 Bi Tuiiio et ah [451 May 1, 1973 [54] HNDEEPENDENTLY ADJUSTAEILE EUAL PULARHZED fiKPLEXER [75] Inventors: Joseph G. Di Tullio, Woburn, Mass; Donald J. Summers, Brookline, N.H.; Windsor D. Wright, Cambridge, Mass.

[73] Assignee: GTE Sylvania Incorporated, New

York,N.Y.

[22] Filed: Aug. 17, 1972 [21] Appl. N0.: 281,522.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Att0mey-Norman J. OMalley et al.

[ 57] ABSTRACT Microwave diplexer apparatus for handling two independent polarized transmitted signals at one frequency and two independent received signals at a lower frequency. The two transmitted signals are introduced separately into a first two-port orthomode transducer so as to be mutually orthogonal. From the orthomode transducer they pass through a polarization rotator, which can be rotated to adjust their angle of polarization, to a first four-port orthomode transducer. The transmitted signals pass through an antenna port of the four-port orthomode transducer to the antenna. Received orthogonally polarized signals from the antenna pass into the four-port orthomode transducer. They are blocked from entering the first two-port orthomode transducer since they are below the cutoff frequency of that transmission path. They pass through four ports which are positioned orthogonally in the four-port orthomode transducer. Filters at these ports prevent the high frequency transmitted signal from passing through. The received signals are conducted by four sections of rectangular waveguide to a second four-port orthomode transducer. The orthogonally polarized received signals pass through a polarization rotator to a second two-port orthomode transducer. When the rotator is properly oriented, the two polarized received signals are separately and independently coupled to two ports of the two-port orthomode transducer which are mutually orthogonal. By separately adjusting the angles of polarization by means of the two polarization rotators, the polarizations of the transmitted and received signals can be changed independently of each other without changing the physical orientation of the apparatus.

6 Claims, 1 Drawing Figure INDEPENDENTLY ADJUSTABLE DUAL POLARIZED DIPELEXER BACKGROUND OF THE INVENTION This invention relates to microwave diplexers. More particularly, it is concerned with diplexer apparatus for handling simultaneously two transmitted signals and two received signals from a single antenna.

One type of communication system includes a master station and a remote station each having a transmitter and a receiver to form a two-way communication link. Typically, the information capacity of the link is governed by the amount of available bandwidth By employing two signals which are mutually orthogonally polarized for each frequency, it is possible to double the number of communication channels available. A diplexer apparatus for handling two orthogonally polarized transmitted signals at one frequency and two orthogonally polarized received signals at another frequency is described and claimed in application Ser. No. 195,41 3, filed Nov. 3, 1971, by Joseph G. DiTullio, Leonard I. Parad, and Kenneth E. Story entitled Dual Polarized Diplexer and assigned to the assignee of the present application.

Although the apparatus as described in the aforementioned application is satisfactory in many communication systems, under certain conditions it is desirable to be able to adjust the polarization of the transmitted and received signals independently in order to change the orientation of either or both with respect to the apparatus. This capability is particularly important in situations where signal propagation effects such as Faraday rotation disturb the polarization of signals in the receive frequency band to a different extent than that of signals in the transmit frequency band. Fixed orthogonal polarization diplexers of the type previously available cannot accommodate the variations of polarization angles caused by the effects of Faraday rotation.

SUMMARY OF THE INVENTION A polarized signal communication system having the capability of permitting independent adjustment of the polarization angles of the transmit and receive signals is provided by employing diplexer apparatus in accordance with the present invention. The diplexer apparatus comprises electromagnetic wave conducting means having first, second, third, and fourth signal ports. The electromagnetic wave conducting means permits passage through the first port of the first and second signals in a first frequency band which are orthogonally polarized with respect to each other, and third and fourth signals in a second frequency band which are orthogonally polarized with respect to each other. It also permits passage of the first and second signals through the second port, and blocks passage of the third and fourth signals through the second port. The electromagnetic wave conducting means permits passage through its third port of the third signal and blocks passage through the third port of the first, second, and fourth signals. It also permits passage through the fourth port of the fourth signal and blocks passage through the fourth port of the first, second, and third signals.

The diplexer apparatus also includes a first transducer means which has first, second, and third ports.

The first port passes signals of the first frequency, and the second and third ports pass two signals of the first frequency which are orthogonally polarized with respect to each other, one of the signals passing through each of the two ports. A second transducer means has first, second, and third ports. The first port passes signals of the second frequency, and the second and third ports pass two signals of the second frequency which are orthogonally polarized with respect to each other, one of the signals passing through each of the ports.

The diplexer apparatus also includes a first polarization adjusting means which is coupled between the second port of the electromagnetic wave conducting means and the first port of the first transducer means. The first polarization adjusting means may be employed to adjust the polarization of signals which pass through it. A second polarization adjusting means is coupled between the third and fourth ports of the electromagnetic wave conducting means and the first port of the second transducer means. The second polarization adjusting means may be employed to adjust the polarization of signals which pass through it.

BRIEF DESCRIPTION OF THE DRAWING Additional objects, features, and advantages of diplexer apparatus in accordance with the present invention will be apparent from the following detailed discussion together with the accompanying drawing wherein the single FIGURE is a pictorial representation of a diplexer apparatus in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Diplexer apparatus in accordance with the present invention as illustrated in the drawing handles two mutually orthogonally polarized transmitted signals at a first frequency and two mutually orthogonally polarized received signals at a second frequency in conjunction with a single antenna. For purposes of discussion in the present example, the transmitted signals are in the higher frequency band and the received signals are in the lower frequency band. As is well understood, the diplexer apparatus operates on the principal of reciprocity and the transmit and receive functions may be reversed without necessitating changes in the diplexer apparatus itself.

The transmitted signals originate from two separate independent transmitters (not shown) and are applied to a two-port orthomode transducer 10 through two separate sections of rectangular waveguide 11 and 12. The two sections of rectangular waveguide 11 and 12 direct the transmitted signals into the section of circular waveguide of the transducer 10 where their electric fields are established at right angles to each other as is well-known in the art. The polarized transmitted signals pass from the two-port orthomode transducer 10 through a polarization rotator 15 and an impedance matching transformer 16 to one port at one end of a four-port orthomode transducer 13. The signals pass through an antenna port at the opposite ends of the four-port orthomode transducer 12 and through an impedance matching transformer M to the antenna (not shown).

A pair of polarized received signals having their eiectric fields orthogonally related are received at the antenna and pass through the transformer lid to enter the four-port orthomode transducer 13 at the antenna port. The received signals are independently coupled from the orthomode transducer 13 through four sections of rectangular waveguide 21, 22, 23, and 24. One vectoral component of each signal divides equally into the first and third sections of rectangular waveguide 2i and and the orthogonal vector component of each signal similarly divides equally into the second and fourth sections of rectangular waveguide 22 and 24-. The orthogonal vector components of each of the two signals are recombined in a second four-port orthomode transducer 26, and pass from there through an adjustable polarization rotator 27 to a second two port orthomode transducer 28. When the polarization rotator 27 is properly oriented, the received signals are independently coupled from the second two-port orthomode transducer 28 through sections of rectangu lar waveguide 29 and 30, which are orthogonal to each other, to their respective receivers (not shown).

As illustrated in the FIGURE, the first four-port orthomode transducer 13 includes a section of circular waveguide having an antenna port or opening at one end coupled to the antenna by way of the impedance matching transformer 14. The opening at the other end is connected to the impedance matching transformer 16. Located in the wall of the section of circular waveguide 13 are four ports to which the four sections of rectangular waveguide 21, 22, 23, and 24 are connected. The four ports are rectangular openings located at orthogonal positions in the waveguide. The long dimensions of the openings as shown in the FIGURE are parallel to the axis of the circular waveguide.

The four rectangular openings are arranged to couple the received signals to the sections of rectangular waveguide. One pair of vectoral components of the two received signals pass through one pair of diametrically opposed openings into the first and third sections of rectangular waveguide 21 and 23, and the other pair of vectoral components pass through the other pair of openings into the second and fourth sections of rectangular waveguide 22 and 24. A pair of diametrically opposed openings rather than a single opening are emplayed for coupling each signal from the circular waveguide in order to maintain symmetry in the circular waveguide and thereby reduce the excitation of higher order modes. A portion 210, 22a, 23a, and 24a of each section of rectangular waveguide 21, 22, 23, and 24 connected to the four-port orthomode transducer 13 is a low pass filter. Each filter produces a short circuit for the higher frequency transmitted signals so that they do not pass into the rectangular waveguide and produces an open circuit or matched impedance for the lower frequency received signals so that they are efficiently coupled to the rectangular waveguide. Details of these low pass filters are described in the aforementioned application of DiTullio et a].

The impedance matching transformer I6 is coupled cuiar waveguide of the polarizer 15 and the two-port orthomode transducer PM. The diameter of the section of circular waveguide of the fourport orthomode transducer 13 is such as to pass both the higher frequency transmitted signals and the lower frequency received signals. The smaller diameter of the circular waveguide of the polarization rotator 15 and two-port orthomode transducer 10 is such as to pass only the higher frequency signals and cut off the lower frequency signals.

The two-port orthomode transducer 10 includes a section of circular waveguide having first and second sections of rectangular waveguide 11 and 12 connected to it at rectangular openings or ports. The openings are oriented with the E planes of the two sections of rectangular waveguide orthogonal to each other thereby establishing the relative polarity of the two independent transmitted signals introduced into the circular section of the two-port orthomode transducer 10 by way of the rectangular waveguide sections 11 and 12.

The polarization rotator 15 is coupled between the transformer 16 and an opening in the end of the orthomode transducer 10. Components of this general type for rotating the polarization of signals within I selected frequency ranges over a complete angular range of 360 are commercially available components. By rotational adjustment, as indicated by the arrow, the orientation of the polarization of the transmitted signals may be adjusted with respect to the elements of structure of the diplexer without changing their position.

The first and third sections of rectangular waveguide 21 and 23 and the second and fourth sections 22 and 24 pass orthogonal vectoral components of each of the two received signals to the second four-port orthomode transducer 26. The filters 21a, 22a, 23a, and 240 block the higher frequency transmitted signals. The pair of orthogonal vectoral components of each received signal are coupled into a large diameter section of circular waveguide of the second four-port orthomode transducer 26 through two pairs of diametrically opposed openings. Each pair of openings is physically orthogonal to the other pair, thus maintaining the polarity of the signals introduced into the circular waveguide.

The polarized signals are passed through an opening at the end of the four-port orthomode transducer 26 to the polarization rotator 27. The polarization rotator 27 is similar to the first polarization rotator 15 except that it operates at the lower frequency. The angular orientation of the signals passing through the rotator 27 is adjusted by rotation of the rotator with respect to the remainder ofthe apparatus as indicated by the arrow in the FIGURE.

The orthogonally polarized received signals pass through the rotator 27 to the second two-port orthomode transducer 28. The two-port orthomode transducer 28 includes a section of circular waveguide of large diameter. The two sections of rectangular waveguide 29 and 30 are connected to the circular waveguide at rectangular openings. The E planes of the two sections of rectangular waveguide 29 and 30 are at right angles so that one of the received signals passes through each section.

The diplexer apparatus as described operates in the following manner in the transmit mode. A first transmitter output signal at the higher frequency is fed into the rectangular waveguide 11 and a second transmitter output signal at the same frequency is fed into the rectangular waveguide 12. These signals are conducted into the section of circular waveguide of the two-port orthomode transducer 10. Because of the symmetry of the circular waveguide portion of the orthomode trans-- ducer and the propagation properties of the rectangular waveguide sections, the two transmitter openings are isolated from each other. Exciting the first section of rectangular waveguide 11 causes an electric field in the circular waveguide which is polarized perpendicular to the broad wall of the first section of rectangular waveguide 11. Similarly, exciting the second section of rectangular waveguide 12 causes an electric field in the circular waveguide which is polarized perpendicular to the broad wall of the second section of waveguide 12. Since the broad walls of these two waveguides are per pendicuiar, the transmitters remain isolated from one another while producing orthogonal fields in the circular section of waveguide of the two-port orthomode transducer ill).

The two orthogonal transmitted signals pass into the polarization rotator 15. The rotator 15 may be rotated so as to adjust the angular orientation of the combined pair of mutually orthogonally polarized signals as desired. The transmitted signals pass from the rotator 15 through the transformer section 16 and into the circular waveguide of the four-port orthomode transducer 13. At the frequency of the transmitted signals, the fil ters 21a, 22a, 23a, and 24a short circuit the openings to the for sections of rectangular waveguide blocking the transmitted signals from the rectangular waveguide. The transmitted signals thus pass from the antenna port in the orthomode transducer 13 to the antenna. The angular orientation of the orthogonally polarized transmitted signals launched from the antenna is controlled by adjusting the polarization rotator 15.

On reception, a pair of orthogonally related signals at the lower frequency are directed from the antenna through the transformer 14 to the antenna port of the four-port orthomode transducer 13. The lower frequency signals are isolated from the transmitter by virtue of the smaller diameter circular waveguide of the polarization rotator l5 and the two-port orthomode transducer 10 which is below cutoff at the received frequency. Thus, when the signals are received in the circular waveguide of the orthomode transducer 13, they leave only through the openings coupling them to the four sections of rectangular waveguide by way of the isolation filters. Vectoral components of the pair of received signals are conducted by combined action of the first and third sections of rectangular waveguide 21 and 23 to the section of circular waveguide of the second four-port orthomode transducer 26. Orthogonal vectoral components of the pair of received signals are connected by the combined action of the second and fourth sections of rectangular waveguide 22 and 24 to the orthomode transducer 26. The signals pass from the orthomode transducer 26 through the polarization rotator 27 to the two-port orthomode transducer 2%. Since the E planes of the two sections of rectangular waveguide 29 and 3d are at right angles, when the rotator 27 is properly oriented, each section of rectangular waveguide propagates a different one of the received signals.

The field orientation for signals in the lower frequency band are identical in the two four-port orthomode transducers l3 and 26. Therefore adjustment of the angles of polarization of the lower frequency hand signals at every point throughout the circular waveguide portions of the apparatus between the transformer 14 and the four-port orthomode transducer 26 is accomplished by rotation of the polarization rotator 27. This capability permits adjustment of the apparatus to compensate for any orientation of the polarized signals received at the antenna without the necessity for making physical adjustment to the diplexer apparatus.

As a specific example, apparatus in accordance with the present invention may be employed with transmitted signals in the frequency range of 5.925 to 6.425 Gl-lz and received signals in the range of 3.7 to 4.2 GHz. The diameter of the small diameter circular waveguide is 1.375 inches and that of the larger diameter circular waveguide is 2.125 inches. As mentioned previously, the higher frequency signals can be the received signals and the lower frequency signals the transmitted signals with no change in the diplexer apparatus as shown and described.

Diplexer apparatus in accordance with the present invention thus provides for continuously variable independent adjustment of the senses of polarization of the transmitted signals and the received signals. The rotational adjustment of polarization over a complete angular range of 360 is accomplished independently for each frequency band solely by the rotational adjustment of the appropriate polarization rotator. The rotators are conventional well-known devices for altering the angle of polarization of signals, and each operates over a relatively narrow band of frequencies. Effects such as Faraday rotation which change the polarization angles of signals at one frequency more than of those at another frequency can therefore be compensated for without any physical changes in the arrangement of the transmitting and receiving equipment.

While there has been shown and described what is considered a preferred embodiment of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined in the appended claims.

What is claimed is:

l. Diplexer apparatus comprising electromagnetic wave conducting means having first,

second, third, and fourth signal ports, said electromagnetic wave conducting means being adapted to permit passage through said first port of first and second signals in a first frequency band which are orthogonally polarized with respect to each other and third and fourth signals in a second frequency band which are orthogonally polarized with respect to each other, being adapted to permit passage through said second port of said first and second signals and to block passage through said second port of said third and fourth signals, being adapted to permit passage through said third port of said third signal and to block passage through said third port of said first, second, and

fourth signals, and being adapted to permit passage through said fourth port of said fourth signal and to block passage through said fourth port of said first, second, and third signals;

first transducer means having first, second, and third ports, said first port being adapted to pass signals of said first frequency, said second and third ports being adapted to pass two signals of said first frequency which are orthogonally polarized with respect to each other, one of the signals passing through each of the ports;

second transducer means having first, second, and third ports, said first port being adapted to pass signals of said second frequency, said second and third ports being adapted to pass two signals of said second frequency which are orthogonally polarized with respect to each other, one of the signals passing through each of the ports;

first polarization adjusting means coupled between the second port of the electromagnetic wave conducting means and the first port of the first trans ducer means and operable to adjust the polarization of signals passing therethrough; and

second polarization adjusting means coupled between the third and fourth ports of the electromagnetic wave conducting means and the first port of the second transducer means and operable to adjust the polarization of signals passing therethrough.

2. Diplexer apparatus in accordance with claim 1 wherein said electromagnetic wave conducting means includes a first section of circular waveguide having an opening at one end corresponding to said first port, an opening at the other end corresponding to said second port, a first rectangular opening in the wall thereof corresponding to said third port, and a second rectangular opening in the wall thereof corresponding to said fourth port, said rectangular openings being located at orthogonal positions in the circular waveguide.

3. Diplexer apparatus in accordance with claim 2 wherein said first transducer means includes a second section of circular waveguide of smaller diameter than said first section having an opening at one end corresponding to said first port and having first and second rectangular openings arranged to pass two signals which are orthogonally polarized with respect to each other, said second section of circular waveguide being adapted to cut off signals having a frequency below that of signals in the first frequency band;

said first polarization adjusting means includes a polarization rotator having one end connected to the opening at the one end of said second section of circular waveguide;

and including impedance matching means coupled between the opening at the other end of said first section of circular waveguide and the other end of said first polarization adjusting means and being adapted to match the impedance between the first and second sections of circular waveguide.

4. Diplexer apparatus in accordance with claim 3 including a third section of clrcular waveguide of the same diameter as said first section and having an opening at one end, first and second rectangular openings in the walls thereof located at orthogonal positions in the circular waveguide;

a first section of rectangular waveguide coupling the first rectangular opening in the first section of circular waveguide and the first rectangular opening in the third section of circular waveguide;

a second section of rectangular waveguide coupling the second rectangular opening in the first section of circular waveguide and the second rectangular opening in the third section of circular waveguide;

and wherein said second transducer means includes a fourth section of circular waveguide of the same diameter as said first and third sections and having an opening at one end corresponding to said first port and having first and second rectangular openings arranged to pass two signals which are orthogonally polarized with respect to each other; and

said second polarization adjusting means includes a polarization rotator having one end connected to the opening at the one end of said fourth section of circular waveguide and the other end connected to the opening at the one end of said third section of circular waveguide.

5. Diplexer apparatus in accordance with claim 4 wherein said first section of circular waveguide has a third rectangular opening in the wall thereof located diametrically opposite said first rectangular opening and a'fourth rectangular opening in the wall thereof located diametrically opposite said second rectangular opening;

said third section of circular waveguide has a third rectangular opening in the wall thereof located diametrically opposite said first rectangular opening and a fourth rectangular opening in the wall thereof located diametrically opposite said second rectangular opening;

and including a third section of rectangular waveguide coupling the third rectangular opening in the first section of circular waveguide and the third rectangular opening in the third section of circular waveguide; and

a fourth section of rectangular waveguide coupling the fourth rectangular opening in the first section of circular waveguide and the fourth rectangular opening in the third section of circular waveguide.

6. Diplexer apparatus in accordance with claim 5 including four filters, one coupled between each of the four rectangular openings in said first section of circu lar waveguide and the associated section of rectangular waveguide, each filter being adapted to present a short circuit across its corresponding opening to signals in the first frequency band and to present a matched impedance between its corresponding opening and the associated section of rectangular waveguide to signals in the second frequency band. 

1. Diplexer apparatus comprising electromagnetic wave conducting means having first, second, third, and fourth signal ports, said electromagnetic wave conducting means being adapted to permit passage through said first port of first and second signals in a first frequency band which are orthogonally polarized with respect to each other and third and fourth signals in a second frequency band which are orthogonally polarized with respect to each other, being adapted to permit passage through said second port of said first and second signals and to block passage through said second port of said third and fourth signals, being adapted to permit passage through said third port of said third signal and to block passage through said third port of said first, second, and fourth signals, and being adapted to permit passage through said fourth port of said fourth signal and to block passage through said fourth port of said first, second, and third signals; first transducer means having first, second, and third ports, said first port being adapted to pass signals of said first frequency, said second and third ports being adapted to pass two signals of said first frequency which are orthogonally polarized with respect to each other, one of the signals passing through each of the ports; second transducer means having first, second, and third ports, said first port being adapted to pass signals of said second frequency, said second and third ports being adapted to pass two signals of said second frequency which are orthogonally polarized with respect to each other, one of the signals passing through each of the ports; first polarization adjusting means coupled between the second port of the electromagnetic wave conducting means and the first port of the first transducer means and operable to adjust the polarization of signals passing therethrough; and second polarization adjusting means coupled between the third and fourth ports of the electromagnetic wave conducting means and the first port of the second transducer means and operable to adjust the polarization of signals passing therethrough.
 2. Diplexer apparatus in accordance with claim 1 wherein said electromagnetic wave conducting means includes a first section of circular waveguide having an opening at one end corresponding to said first port, an opening at the other end corresponding to said second port, a first rectangular opening in the wall thereof corresponding to said third port, and a second rectangular opening in the wall thereof corresponding to said fourth port, said rectangular openings being located at orthogonal positions in the circular waveguide.
 3. Diplexer apparatus in accordance with claim 2 wherein said first transducer means includes a second section of circular waveguide of smaller diameter than said first section having an opening at one end corresponding to said first port and having first and second rectangular openings arranged to pass two signals which are orthogonally polarized with respect to each other, said second section of circular waveguide being adapted to cut off signals having a frequency below that of signals in the first frequency band; said first polarization adjusting means includes a polarization rotator having one end connected to the opening at the one end of said second section of circular waveguide; and including impedance matching means coupled between the opening at the other end of said first section of circular waveguide and the other end of said first polarization adjusting means and being adapted to match the impedance between the first and second sections of circular waveguide.
 4. Diplexer apparatus in accordance with claim 3 including a third section of circular waveguide of the same diameter as said first section and having an opening at one end, first and second rectangular openings in the walls thereof located at orthogonal positions in the circular waveguide; a first section of rectangular waveguide coupling the first rectangular opening in the first section of circular waveguide and the first rectangular opening in the third section of circular waveguide; a second section of rectangular waveguide coupling the second rectangular opening in the first section of circular waveguide and the second rectangular opening in the third section of circular waveguide; and wherein said second transducer means includes a fourth section of circular waveguide of the same diameter as said first and third sections and having an opening at one end corresponding to said first port and having first and second rectangular openings arranged to pass two signals which are orthogonally polarized with respect to each other; and said second polarization adjusting means includes a polarization rotator having one end connected to the opening at the one end of said fourth section of circular waveguide and the other end connected to the opening at the one end of said third section of circular waveguide.
 5. Diplexer apparatus in accordance with claim 4 wherein said first section of circular waveguide has a third rectangular opening in the wall thereof located diametrically opposite said first rectangular opening and a fourth rectangular opening in the wall thereof located diametrically opposite said second rectangular opening; said third section of circular waveguide has a third rectangular opening in the wall thereof located diametrically opposite said first rectangular opening and a fourth rectangular opening in the wall thereof located diametrically opposite said second rectangular opening; and including a third section of rectangular waveguide coupling the third rectangular opening in the first section of circular waveguide and the third rectangular opening in the third section of circular waveguide; and a fourth section of rectangular waveguide coupling the fourth rectangular opening in the first section of circular waveguide and the fourth rectangular opening in the third section of circular waveguide.
 6. Diplexer apparatus in accordance with claim 5 including four filters, one coupled between each of the four rectangular openings in said first section of circular waveguide and the associated section of rectangular waveguide, each filter being adapted to present a short circuit across its corresponding opening to signals in the first frequency band and to present a matched impedance between its corresponding opening and the associated section of rectangular waveguide to signals in the second frequency band. 